1. Field of the Invention
The present invention relates to a method of manufacturing a liquid crystal display device (LCD) and more specifically, to a method for manufacturing an LCD including at least two stacked thin layers in which the upper thin film smoothly and completely covers the lower thin film and in which a photo-resist layer is formed in a single masking step to have a thick portion and a thin portion.
2. Description of the Background Art
A thin film type liquid crystal display device includes an upper panel, a lower panel and a liquid crystal material inserted therebetween. At the outer side of the two joined panels, polarizing plates are attached. The upper panel includes an inner side having a color filter and a common electrode and an outer side having a polarizing plate. As seen in FIGS. 1, 2, 3a and 3b, the lower panel includes an outer side having another polarizing plate and an inner side having a plurality of gate bus lines 10 and a gate pad 10a, a plurality of data bus lines 20 and data pad 20a, a TFT switching element C and a pixel electrode 30.
The structure of the lower panel is explained hereafter in detail, referring to FIG. 2 which shows a plan view of the conventional LCD and FIG. 3a which shows a cross-sectional view of the conventional LCD along the line A—A of FIG. 2.
A plurality of the gate bus lines 10 perpendicularly cross a plurality of the data bus lines 20. The TFT switching element C, which includes a gate electrode 11 which is derived from the gate bus line 10, a source electrode 21 which is derived from the data bus line 20 and a drain electrode 22 which faces the source electrode 21, is disposed at the intersection portion of the gate bus line 10 and the data bus line 20. A pixel electrode 30 connected to the drain electrode 22 and an output electrode of the TFT C are formed in the area surrounded by the gate line 10 and the source line 20.
The process of manufacturing the lower panel of the conventional LCD is explained hereinafter.
A first metal layer is formed by depositing aluminum or aluminum alloy on a transparent substrate 1. A plurality of gate bus lines 10, a gate pad 10a and a gate electrode 11 derived from the gate bus line 10 are formed by etching the first metal layer.
A gate insulating layer 12 which has a good adhesive property with an amorphous silicon and a high insulating property, such as SiNx or SiOx, is formed on the substrate 1 which includes the gate bus line 10, the gate electrode 11 and the gate pad 10a. 
On the gate insulating layer 12, an amorphous silicon and an n+ type impurity doped amorphous silicon are sequentially deposited and patterned to form an intrinsic semiconductor layer 15 and a doped semiconductor layer 16 (or an ohmic contact layer).
A second metal layer is formed on the entire surface of the substrate, and may be formed of aluminum or aluminum alloy. The second metal layer is patterned to form a plurality of data bus lines 20 which perpendicularly cross the gate bus lines 10, a data pad 20a which is disposed at the each end of the data bus line 10, a source electrode 21 which is derived from the data bus line 20 and a drain electrode 22 which faces the source electrode 21. As a result, a TFT switching element including the gate electrode 11, the semiconductor layers 15 and 16, the source electrode 21 and the drain electrode 22 is completed.
On the substrate including the gate bus line 10, the data bus line 20, the gate pad 10a, the data pad 20a and the switching element, a passivation layer 23 is formed using SiNx, SiOx or BCB(benzocyclobutene). A contact hole is formed by removing some portion of the passivation layer 23 which covers the drain electrode 22 to expose some portions of the drain electrode 22.
An ITO(Indium Tin Oxide) layer is deposited on the passivation layer 23 via a sputtering method. The pixel electrode 30 is formed by patterning the ITO layer. The pixel electrode 30 is connected to the drain electrode 22 through the contact hole.
The method of manufacturing the conventional LCD includes many processes for forming thin layers which are stacked on each other, and the thin layers are deposited and patterned via masking processes. The LCD according to this conventional method has a stacked structure as shown in FIG. 3a in which the gate bus line 10 and the data bus line 20, the gate electrode 11 and the data electrode 21, the pixel electrode 30 and the drain electrode 22 cross each other.
In this stacked structure, the cross-sectional shape of the lower layer is a main factor for determining the deposited state of the upper layer. If the cross sectional shape of the lower layer has an inverse tapered shape or a shoulder, the upper layer deposited thereon has discontinued or unstable portions.
For example, as shown in FIG. 3b, the cross-sectional shape, taken along the line B—B of FIG. 2, of the drain electrode 22 determines how the passivation layer 23 and the pixel electrode 30 will be deposited thereon. When the cross-sectional shape of the drain electrode 22 has an inverse tapered shape, the passivation layer 23 has a shoulder 24 or crack formed therein. At these portions having the shoulder 24 or the cracks, the pixel electrode 30 when deposited has a greatly reduced thickness or is even discontinued at this portion. Furthermore, when the pixel electrode is patterned by using an etchant on the cracked passivation layer, the drain electrode can be damaged by the etchant as it spreads or percolates through the cracks.